The present invention relates to polyhalide particles of improved stability for use in light valves and in set suspensions and to a method of making such particles.
Light valves have been known for over sixty years for modulation of light. As used herein, a light valve may be described as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains a light-modulating element, which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed and encapsulated.
The liquid suspension (sometimes herein called a liquid light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
Light valves have been proposed for many purposes including, e.g., alpha-numeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing there through. Light valves of the type described herein are also known as xe2x80x9csuspended particle devicesxe2x80x9d or xe2x80x9cSPDsxe2x80x9d.
For many applications, it is preferable for the activatable material to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects e.g., bulging, associated with a high column of liquid suspension can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film the particles are generally present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A xe2x80x9clight valve filmxe2x80x9d as used herein is thus a film having droplets of a liquid suspension of particles distributed in the film.
A type of light valve film made by phase separation from a homogeneous solution is disclosed in U.S. Pat. No. 5,409,734. Light valve films made by cross-linking emulsions are disclosed in U.S. Patent Nos. 5,463,491 and 5,463,492 assigned to the assignee of the present invention. All of those patents and other patents and articles cited herein are incorporated herein by reference.
For use in set suspensions such as light-polarizing sheets, sometimes called xe2x80x9csheet polarizersxe2x80x9d, which can be cut up and formed into polarized sunglass lenses or used as filters, light-polarizing particles can be dispersed or distributed throughout a sheet of suitable film-forming material, such as cellulose acetate or polyvinyl alcohol or the like. Methods of making set suspensions for use in sheet polarizers are well known in the prior art. In these set suspensions, however, the particles are immovable. See e.g., U.S. Pat. Nos. 2,178,996 and 2,041,138.
A. Liquid Suspending Media and Stabilizers
The liquid light valve suspension for use with the particles obtained from the method of the present invention may be any liquid light valve suspension known in the art and may be formulated according to known techniques. The term xe2x80x9cliquid light valve suspensionxe2x80x9d as used herein means a xe2x80x9cliquid suspending mediumxe2x80x9d in which a plurality of small particles is dispersed. The xe2x80x9cliquid suspending mediumxe2x80x9d comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspension of the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art are useful herein, such as but not limited to the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer when employed, can be a single type of solid polymer that bonds to the surface of the particles but also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose, which in effect, provides a plain surface coating for the particles and one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
B. Particles
As is known, inorganic and organic particles may be used in a light valve suspension. However, the present invention relates to an improved method of preparing particles that are polyhalides (sometimes referred to in the prior art as perhalides) of alkaloid acid salts and the like. The polyhalide particles of the present invention may be light-polarizing, such as halogen-containing light-polarizing materials, e.g., polyhalides of alkaloid acid salts. (The term xe2x80x9calkaloidxe2x80x9d is used herein to mean an organic nitrogenous base, as defined in Hackh""s Chemical Dictionary, Fourth Edition, McGraw-Hill Book Company, New York, 1969). As is known, if a polyhalide of an alkaloid acid salt is prepared, the alkaloid moiety may be a quinine alkaloid, as defined in Hackh""s Chemical Dictionary, supra. U.S. Pat. Nos. 2,178,996 and 2,289,712 refer in detail to the use of polyhalides of quinine alkaloid acid salts. The particles may be light-absorbing or light-reflecting.
Also, the particles of the present invention may be a hydrogenated polyhalide of a quinine alkaloid acid salt, such as dihydrocinchonidine sulfate polyiodide, as described in U.S. Pat. No. 4,131,334.
More recently, improved polyhalide particles having advantageous features for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These xe2x80x9cpolyhalide particlesxe2x80x9d are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohalide acid or an ammonium halide, alkali metal halide or alkaline earth metal halide. These xe2x80x9cpolyhalide particlesxe2x80x9d can be advantageously prepared by the method of the present invention.
Prior art polyhalide particles are also discussed in detail in xe2x80x9cThe Optical Properties and Structure of Polyiodidesxe2x80x9d by D. A. Godina and G. P. Faerman published in The Journal of General Chemistry, U.S.S.R. Vol. 20 pp. 1005-1016, (1950). Herapathite, for example, is quinine bisulfate polyiodide, and its formula is given under the heading xe2x80x9cquinine iodosulfatexe2x80x9d as 4C20H24N2O2.3H2SO4.2HI.I4.6H2O in The Merck Index, 10th Ed. (Merck and Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodine anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 (1978), pp. 3215-3217. The term xe2x80x9cpolyhalidexe2x80x9d is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion.
As is known, polyhalide particles that are useful for light valves are preferably of colloidal size, that is the particles will have a large dimension averaging about 1 micron or less. It is preferred that most polyhalide particles have large dimensions less than one-half of the wavelength of blue light, i.e. 2000 Angstroms or less to keep light scatter extremely low.
For intended commercial uses it is necessary that the polyhalide particles used in a liquid light valve suspension, whether or not incorporated into a film, have great chemical and environmental stability. Historically, the early liquid light valve suspensions comprised particles of Herapathite, referred to above. However, Herapathite and closely related compounds had poor stability to many chemicals and degraded readily when exposed to either ultraviolet radiation or high temperatures. Some improvement in stability was observed for polyhalide particles made from salts of certain alkaloids which, unlike quinine, had been hydrogenated and did not include a methoxy group. See e.g., U.S. Pat. No. 4,131,334. Other types of polyhalide particles having still better heat stability and not based on alkaloids were disclosed in U.S. Pat. Nos. 4,877,313 and 5,002,701.
Also, U.S. Pat. No. 5,516,463 discloses, inter alia, greatly improved light-polarized polyiodide particles formed by reacting iodine and calcium iodide with the compound pyrazine 2,5-dicarboxylic acid dihydrate. Although this polyhalide particle represented a significant advance over prior art particles for use in light valves, its chemical stability was still less than optimum. For example, polyhalide particles made from pyrazine-2,5-dicarboxylic acid degrade over a period of time when contacted with water. This degradation is evidenced by the formation of an off-white solid precipitate when the polyhalide particle is soaked in water.
Accordingly, a yet more stable type of polyhalide light-polarizing particle is desirable for uses where a high degree of chemical and/or environmental stability is needed, especially in light-polarizing sheets and in liquid light valve suspensions and light valve films for light valves.
The present invention comprises polyhalide light-polarizing particles having improved properties, and liquid and set suspensions and films comprising the same. These improved particles are made by forming a complex of (a) elemental iodine, (b) a hydrohalide acid or an ammonium halide, alkali metal halide or alkaline earth metal halide, (c) a first compound capable of chelating hydrogen, ammonium, or metal ions (said compound hereinafter sometimes called the xe2x80x9cPrecursorxe2x80x9d) and (d) a second compound comprising at least one chelating group also present in the Precursor and otherwise structurally identical to the Precursor except that in the second compound either (1) at least one group of the Precursor is changed to a different group or (2) the second compound comprises at least one additional group (said second compound hereinafter sometimes called the xe2x80x9cModified Precursorxe2x80x9d). Any amount of Modified Precursor that is effective can be used, up to about 100 mol % of the Precursor.
As used herein, as between the above mentioned first and second compounds, if a greater quantity of one compound than the other is used is forming the complex, the compound which is present in greater amount relative to the other, measured in mol percent, shall be deemed to be the Precursor, and the other compound the Modified Precursor. If the compounds are present in equal mol percents, then either compound may be deemed to be the Precursor and the other compound the Modified Precursor.
The Precursor may be any of the compounds previously used to form organic polyhalide particles by reaction with elemental iodine and a hydrohalide acid or an ammonium, alkali metal or alkaline earth metal halide. For example, the Precursor may be a quinine alkaloid acid salt (U.S. Pat. Nos. 2,178,996 and 2,289,712), a hydrogenated alkaloid acid salt (U.S. Pat. No. 4,131,334) or an organic compound containing one or more groups that chelate hydrogen, ammonium or metal ions (U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463), all of such U.S. patents being incorporated herein by reference thereto.
It is presently preferred to use as the Precursor, pyrazine-2,5-dicarboxylic acid. (U.S. Pat. No. 5,516,463), and to use as the Modified Precursor, the mono-alkyl ester or the monoamide of pyrazine-2,5-dicarboxylic acid, most preferably in the form of the dihydrate of the diacid Precursor and the monohydrate of the monoacid Modified Precursor, respectfully. The alkyl moiety of the monoester may preferably contain from 1 to about 20 carbon atoms. The nitrogen atom of the monoamide may be substituted by one or two alkyl groups, each preferably of from 1 to about 10 carbon atoms.
Preferably, in forming a given type of polyhalide particle, the amount of Modified Precursor used will range from about 0.01 mol % to about 10 mol % of the amount of the Precursor more preferably, from about 0.1 mol % to about 5 mol %, and most preferably from about 0.2 mol % to about 3 mol % of the amount of the Precursor.
For many of the polyhalide particles of the present invention the changed or additional groups which the Modified Precursor comprises are non-polymeric. However, such groups may also be polymeric, and thus such group or groups in a given Modified Precursor may be either polymeric, non-polymeric or a mixture thereof.
In the case where the Modified Precursor comprises a group changed so as to be different from a group in the Precursor, said changed group in the Modified Precursor may be either larger or smaller than the group in the Precursor. For example, if the Precursor comprises a carboxyl group, the Modified Precursor could have in its place a smaller group such as a halide atom or a methyl group or, alternatively, a larger group such as an alkyl ester or a dialkyl amide group. In some cases it is possible to chemically modify the Precursor to change the group and form the Modified Precursor from the Precursor; in other cases the Modified Precursor, including the changed group, may be synthesized by any other suitable procedure known in the art. As indicated above, the term xe2x80x9cgroupxe2x80x9d as used herein can be as small as one atom. Hence if the Precursor is pyrazine-2,5-dicarboxylic acid, the compound pyridine-2,5-dicarboxylic acid could be considered a Modified Precursor for it, even though the only change in structure would be substitution of a carbon atom for a nitrogen atom in the ring.
When changed groups in the Modified Precursor are large compared to the corresponding groups in the Precursor, it has been observed that polyhalide particles made according to the present invention tend to have a smaller average size than prior art polyhalide particles made under similar conditions without Modified Precursor. Thus we have concluded that the Modified Precursor can act as a crystal growth inhibitor, and that such Modified Precursor molecules are likely to be located on the crystal surfaces. Thus it is readily possible to modify the surface properties of polyhalide particles by selecting a suitable Modified Precursor.
Based on our observations of numerous instances where all of the Precursor and Modified Precursor appear to have been used up in forming polyhalide light-polarizing crystals, we have concluded that the Precursor and Modified Precursor co-crystallize along with the halide and elemental iodine that are present when the crystalline particles of this invention are formed. This conclusion is also consistent with the fact that some particles of the present invention exhibit significantly different properties such as enhanced stability with respect to water and ultraviolet radiation, described hereinafter, compared to the properties of prior art polyhalide particles formed without Modified Precursor.
As discussed above, a wide variety of Precursors are usable in the present invention, such as those disclosed in the prior art. Thus the Precursor can be of any type and may, for example, comprise aromatic, aliphatic and/or aralkyl groups or fused rings, and the changed or additional groups in the Modified Precursor can be of any type, provided that such group does not prevent formation of stable polyhalide particles or, where intended for use in a liquid or set suspension, does not render the particles soluble in the suspending medium.